ci(compat): restore minimum-Zsh and latest-Zi integration coverage #97

Context

PR #76 added an explicit Ubuntu 22.04/Zsh 5.8.1 assertion and scheduled latest-Zi-managed integration coverage. The large #61 integration replaced those workflow files. Current CI still runs the full suite in an Ubuntu 22.04 container, but it no longer asserts the intended minimum version and no longer exercises the latest Zi-managed build/load path.

zpmod is shipped as a binary module and consumed through Zi, so both compatibility boundaries should be tested intentionally rather than inferred from an image tag.

Parent governance issue: #70.

Required work

• Define the supported minimum Zsh version in repository documentation.
• Assert and run the full suite against that exact minimum.
• Restore a scheduled latest-Zi-managed build and zmodload smoke path.
• Exercise at least one generated/published package installation, not only source-tree builds.
• Record the intended ABI compatibility policy for prebuilt artifacts across Zsh versions and platforms.

Acceptance criteria

• CI fails if the minimum-version job silently changes Zsh version.
• The standalone and Zi-managed paths both build and load the candidate revision.
• Scheduled integration records the Zi revision under test.
• Package-install smoke coverage verifies the installed module and completion paths.
• Documentation and workflow behavior agree on the support floor.